On Wed, May 5, 2021 at 12:43 PM Matthias van de Meent <boekewurm+postg...@gmail.com> wrote: > I believe that it cannot be "just" an additive thing, at least not > through a normal INCLUDEd column, as you'd get duplicate TIDs in the > index, with its related problems. You also cannot add it as a key > column, as this would disable UNIQUE indexes; one of the largest use > cases of global indexes. So, you must create specialized > infrastructure for this identifier.
You're just quibbling about the precise words that I used. Of course it is true that there must be some sense in which a global index partition key attribute will need to be special to the implementation -- how else could a global index enforce uniqueness? That was clearly implied. > And when we're already adding specialized infrastructure, then this > should probably be part of a new TID infrastructure. This is a non-sequitur. > And if we're going to change TID infrastructure to allow for more > sizes (as we'd need normal TableAM TIDs, and global index > partition-identifying TIDs), I'd argue that it should not be too much > more difficult to create an infrastructure for 'new TID' in which the > table AM supplies type, size and strict ordering information for these > 'new TID's. > > And if this 'new TID' size is not going to be defined by the index AM > but by the indexed object (be it a table or a 'global' or whatever > we'll build indexes on), I see no reason why this 'new TID' > infrastructure couldn't eventually support variable length TIDs; or > constant sized usertype TIDs (e.g. the 3 int columns of the primary > key of a clustered table). You're not considering the big picture. It's not self-evident that anybody will ever have much use for a variable-width TID in their table AM, at least beyond some fairly simple scheme -- because of the fundamental issue of TID not working as a stable identifier of logical rows in Postgres. If it was very clear that there would be *some* significant benefit then the costs might start to look reasonable. But there isn't. "Build it and they will come" is not at all convincing to me. -- Peter Geoghegan
